Materials and Design 50 (2013) 370–375

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Materials and Design

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Technical Report

Microstructure and tensile strength of friction stir welded jointsbetween interstitial free steel and commercially pure aluminium

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.02.017

⇑ Corresponding author at: Department of Metallurgy and Materials Engineering,Bengal Engineering and Science University, Shibpur, Howrah 711 103, India. Tel.:+91 (033) 26684561/63; fax: +91 (033) 26682916.

E-mail addresses: skundu@metal.becs.ac.in, skundu@mines.edu (S. Kundu).

S. Kundu a,c,⇑, D. Roy b, R. Bhola c, D. Bhattacharjee b, B. Mishra c, S Chatterjee a

a Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711 103, Indiab Research and Development, Tata Steel, Jamshedpur 831 007, Indiac Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401, USA

a r t i c l e i n f o

Article history:Received 13 June 2012Accepted 7 February 2013Available online 26 February 2013

a b s t r a c t

In the present study, the joining of interstitial free steel and commercial pure aluminium was carried outby friction stir welding (FSW) technique using tool rotational speeds of 600, 900, 1200 rpm and traversespeed of 100 mm/min. The microstructure and micro-hardness of the weld interface have been investi-gated. Optical microscopy was used to characterize the microstructures of different regions of friction stirwelding joints. The scanning electron microscopy-back scattered electron (SEM-BSE) images show theexistence of the different reaction layers in the welded zone. The Al3Fe intermetallic compound has beenobserved in the weld interface and their thickness increase with the increase in tool rotational speed.Tensile strength was also evaluated and maximum tensile strength of �123.2 MPa along with �4.5%elongation at fracture of the joint have been obtained when processed at 600 rpm tool rotational speed.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

There is an increasing need to make joints of dissimilar materi-als of complex shaped primary and secondary components forindustrial applications. The sound joints between dissimilar mate-rials enable to multi-materials design methodologies and eco-nomic fabrication process to be employed [1]. The developmentof the automotive industry needs component weights to bereduced in order to improve the performance of automotive vehi-cles [1,2]. In this regard aluminium/aluminium alloy and steel arecommonly used in automotive industry due to specific light weightand cost effectiveness. Thus the problem of welding for these dis-similar materials must be faced. It is known that a variety of at-tempts like laser welding or conventional arc welding to jointhese dissimilar materials may face severe problems like distortionin shape, cracking, formation of intermetallics or brittle cast struc-ture at the interface [3,4].

Friction stir welding (FSW) is one of the most appropriate weld-ing techniques for joining dissimilar materials [5–7]. The joiningtakes place through the movement of a rotating shouldered toolwith a profiled pin plunged into the joint line between two piecesof sheet or plate material. When the rotating pin tool moves alongthe weld line, the material is heated by the friction produced be-

tween the shoulder of the tool and the work piece to be welded.Frictional heat causes the material to soften without reaching themelting point [8,9]. FSW is a solid state welding process for gener-ating reproducible high quality welds of similar and dissimilarmaterials [10–13]. Friction stir welding has already been used forlow melting point materials like Al, Mg and Cu and their alloys[14–16]. Literatures concerning steel vs. steel or steel vs. Al alloyby friction stir welding are scanty [10,17–21]. Lee et al. [1] hasreported that friction stir welding between stainless steel and Alalloy at 800 rpm tool rotational speed and 80 mm/min tool travelspeed. This study also reports that the 250 nm thick Al4Fe interme-tallic compound was identified at the weld interface. They have notevaluated the bond strength of joints. Watanabe et al. [22]reported that friction stir welding of Al alloy and mild steel pro-vides the maximum bond strength of �86% of that of Al alloy,when processed using tool rotating speed of 100 to 1250 rpmand travel speed of 25 mm/min and steel was used at the advanc-ing side. Geiger et al. [2] also studied the friction stir knead weldedof DC04 and Al alloys Butt joints at the tool rotational speed of400–1250 rpm and 1–3� tool tilt angle. Uzun et al. [9] has reportedthat Al 6013-T4 to X5CrNi18-10 stainless steel friction stir weldedjoint was produced at 800 rpm tool rotating speed and 80 mm/mintravel speed and the stainless steel was used at the advancing side.

In the present study efforts were given to produce joint of inter-stitial free steel and commercially pure aluminium by friction stirwelding with varying tool rotational speeds. The investigation alsofocuses on the characteristics of interfacial microstructure and thestrength properties of the joints.

Table 1Tensile properties of the base metals at room temperature.

Alloys 0.2% Proof stress(MPa)

Ultimate tensilestrength (MPa)

Elongation atFracture (%)

CP Al 106.0 132.1 9.8IF steel 135.2 287.2 49

Fig. 1. Schematic diagram of the friction stir welding: (a) diagram of processing, (b)tool dimension, (c) tool position in the interface of materials.

S. Kundu et al. / Materials and Design 50 (2013) 370–375 371

2. Experimental details

The commercially pure aluminium (CP Al) and interstitial freesteel (IF steel) were friction stir welded using a FSW machine.The dimensions of plates were 140 mm � 70 mm � 3 mm. Thechemical composition of commercially pure aluminium is Al–0.02Fe–0.2Si–0.003Mn–0.002S–0.002P–0.005 N (wt%) and that ofinterstitial free steel is Fe–0.004C–0.05Mn–0.006Si–0.007S–0.008P–0.002 N–0.03Ti (wt%). The room temperature mechanicalproperties of commercially pure aluminium and interstitial freesteel are given in Table 1. Welding was carried out along rollingdirection. The pin of friction stir welding tool was made of WCmaterial and shoulder was made of high speed steel. A schematicdiagram of FSW with samples location and tool dimension isshown in Fig. 1.

The high speed steel having 25 mm diameter shoulder and con-ical pin of 5 mm diameter and 2.7 mm height was used for frictionstir welding (Fig. 1b). Normal load of �5 kN and tool tilt angle (2�)were kept constant during processing. The tool rotational speedwere 600, 900, 1200 rpms and travel speed was 100 mm/min.The welding direction of IF steel and CP Al were parallel to the roll-ing direction of the plates. IF steel and CP Al were at advancing andretreating side respectively and were clamped rigidly over the bedfrom four sides to avoid vibration or displacement during process-ing. Unlike the conventional friction stir welding, the tool pin wasplaced towards the aluminium plate (i.e. �80% of pin dimensionshown in Fig. 1c). Therefore, the stirring action of the pin took partmainly in the aluminium. This was done to prevent pin erosion andover-heating of the aluminium [1,9].

From the welded assemblies, a transverse section was taken andsurfaces were prepared by conventional metallographic technique.Both base metals and welded specimens were prepared by conven-tional metallographic technique. Kellers’ reagent and 5% nital wereused for etching regent of CP Al and IF steel respectively. Afteretching, macro and microstructure of the bonded samples were ob-served in a light microscope. The polished surface of the weldedcouples was also examined in a scanning electron microscope

Fig. 2. (a) Surface view of the welded joints (defect marked by arrows), (b) macrostructure of the processed at 600 rpm, (c) macro structure of the joints processed at1200 rpm.

372 S. Kundu et al. / Materials and Design 50 (2013) 370–375

(JEOL JSM-5510) in back scattered mode (SEM-BSE) to obtain finerstructural details of the interface of FSW joints. The chemical com-position of the reaction layers was determined in atomic percent-age by energy dispersive spectroscopy (NORAN System Six).Tensile properties of the welded joints were evaluated in a tensiletesting machine (Instron 4204) at a crosshead speed of1.66 � 10�3 mm/s at room temperature. Sub-size tensile speci-mens with reduced grip dimension were prepared as per ASTMID: E8M-11 , keeping the interface at the centre of the gaugelength. Four samples were tested at each process parameter tocheck the reproducibility of results. The polished transversewelded samples for micro-hardness measurement were carriedusing a diamond micro-indenter with a 20 gf load for 15 s duration.Fracture surfaces of the samples were observed in secondary elec-tron mode in SEM (JEOL JSM-5510).

3. Results and discussion

The surface views of the friction stir welded joints are shown inFig. 2a. At higher rotational speeds, burning defect has been ob-served at surface of the welded joints. The optical macrostructureof the friction stir welded joints were shown in Fig. 2b and c. Light-er side represents aluminium and darker side represents steel. It is

(a)

(c)

(e)Fig. 3. Optical microstructure (a) FSW joint interface at 900 rpm, (b) FSW joint interface aat 900 rpm, (e) TMAZ in Al side at 1200 rpm, (f) HAZ in IF steel side at 900 rpm.

found that second phase particle had been entrapped in aluminummatrix. The optical microstructures of the friction stir weldedjoints are shown in Fig. 3. The interface was clearly reveled inthe weld joints. Stirred zone (SZ) was mainly formed in the Al sideas the welding tool was shifted towards the Al side (Fig. 3c). Brokensteel particles escaped from the surface were distributed withinthe SZ [9]. The SZ has a structure resembling to that of a steel par-ticle reinforced in Al. In the steel side, a metal-flow structure wasobserved within the interface regions. The grain sizes at differentregions of the friction stir welded joints are shown in Fig. 4. Ithas been found that grain size number increases with decrease intool rotating speed in the stir region for both sides. Higher rota-tional speed produced the higher temperature due to friction ascompared to the lower rotational speed [22]. The CP Al side, weldnugget consists of fine, equiaxed, recrystallized grains of approxi-mately 8–10 ASTM number. The fine recrystallized grains in thestirred zone are attributed to the generation of high deformationand temperature during friction stir welding [9,16]. Cho et al.[23] observed the fine grain microstructure in the SZ and TMAZ re-gion, due to continuous dynamic recrystallization in the frictionstir welded joint of high strength line pipe steel.

The SEM-BSE images of friction stir welded joints are shown inFig. 5. At a lower tool rotational speed, crack was not observed at

(b)

(d)

(f)t 1200 rpm, (c) stir zone in Al side at 600 rpm, (d) stir zone and TMAZ in IF steel side

BM HAZ TMAZ SZ SZ TMAZ HAZ BM

7

8

9

10

11

12

13

14

15

CP Al side

600 rpm 900 rpm 1200 rpm

FSW Zones

AST

M G

rain

siz

e nu

mbe

r

IF Steel side

Fig. 4. ASTM grain size numbers of different regions of the friction stir weldedjoints at various tool rotating speeds.

Fig. 6. EDS analysis of reaction layer formed at weld interface.

S. Kundu et al. / Materials and Design 50 (2013) 370–375 373

the joint interface. However crack was observed when sample wasprocessed at 900 and 1200 rpm (marked in Fig. 5c and e). The lightshaded layer has been observed at the weld interface for all the

(a)

(c)

(e)Fig. 5. SEM-BSE images of the friction stir welded joints at 100 mm/min travel speed a1200 rpm (crack marked by arrows).

samples. The concentration of the reaction layer are shown inFig. 6, which was enriched with Al (�59.2–60.3 wt%) and Fe(�39.7–40.8 wt%). Hence, the Al–Fe binary phase diagram indi-cates the formation of Al3Fe intermetallic compounds [22,24].The reaction layer has been formed due to mechanical mixed ofFe and Al during friction stir welding. The thickness of the reactionlayer increases with the increase in tool rotational speed and high-er rotational speed generates higher temperature (Fig. 7). In the Alside very fine light and light shaded particles have been observed.The light particles are IF steel and the light shaded particles are Al3-

Fe intermetallic compounds. Springer et al. [25] investigated theformation of reaction layers which formed at the interface of

(b)

(d)

(f)t various tool rotating speeds of (a) and (b) 600 rpm, (c and d) 900 rpm, (e and f)

600 900 1200

0

5

10

15

20

25

30La

yer w

idth

/ µm

Tool rotational speed / rpm

Fig. 7. Width of intermetallics at the joint interface at various tool rotating speeds.

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

0

100

200

300

400

500

600

700

TMAZTMAZ

Har

dnes

s / H

V

SZSZ

IF SteelCP Al

Distance / mm

600 rpm 900 rpm 1200 rpm

Fig. 8. Micro-hardness of the friction stir welded joints at various tool rotatingspeed.

600 900 1200

80

90

100

110

120

Tensile Sterength Elongation

Tool rotating speed / rpm

Stre

ngth

/ M

Pa

1

2

3

4

5

6E

longation / %

Fig. 9. Tensile properties of the friction stir welded joints processed at various toolrotational speeds.

Fig. 10. Fracture surfaces of the FSW joints processed at (a) 600 rpm, (b) 900 rpm,(c) 1200 rpm tool rotating speed.

374 S. Kundu et al. / Materials and Design 50 (2013) 370–375

FSW joints between low Carbon steel and pure Al/Al–Si (5 wt%) al-loy. They report that in the as-welded state, at the interface, nointermetallic reaction product could be observed, and only afterannealing did the intermetallic compounds make detailed investi-gations possible. Watanabe et al. [22] reported that FeAl and FeAl3

intermetallics were identified in the weld interface, howeveraccording to Sun et al. [26] FeAl6 and Al5Fe2 intermetallics havealso been observed in the dissimilar Al alloy/steel spot FSW jointinterface.

The hardness profiles measured along the transverse cross sec-tion of the friction stir welded joints between CP Al and IF steel areindicated in Fig. 8. At the interface, higher hardness values havebeen obtained due to the presence of intermetallics. The hardnessof the stir zone with an inhomogeneous distribution of light andlight shaded particles depends on the measured point of the hard-ness indenter. Therefore the hardness of the stir zone exhibits var-iable values due to the presence of the fine or coarse dispersed IFsteel particles in the stir zone. The hardness values of the basematerials i.e. CP Al and IF steel are 49 ± 4 and 121 ± 3 HV, respec-tively. However, the hardness values at the stir zone of Al sideare higher as compared to the other regions (�470 ± 10 HV) dueto the presence of Al3Fe intermetallic compounds. Coelho et al.[27] investigated the micro-hardness value of FSW joints betweenhigh strength steel and Al alloy and reported higher hardness of theinterface due to the presence of intermetallics reaction. Literaturereports that the friction stir welding between steel and Al/Al alloy,exhibits variable micro-hardness of the weld nugget due to the

presence of dispersed fine or coarse stainless steel particles. More-over, the micro-hardness at the TMAZ and HAZ region were lowerthan the weld nugget due to the thermomechanical coarsening ofthe dispersed particles [9,28].

The tensile properties of friction stir welded joints with thechange in tool rotational speeds are shown in Fig. 9. A tool rotationspeed of 600 rpm made a good joint, showing the maximum tensilestrength of �123 MPa (about 86% of the CP Al base metal) alongwith �4.5% elongation at fracture. With an increase in the toolrotational speed, the weld strength decreases gradually due tothe increase in the thickness of intermetallics and weld strengthattains the lowest tensile strength (�95.5 MPa) along with �1.5%elongation at fracture, when welding was carried out at1200 rpm tool rotational speeds. At higher rotational speed, highertemperature was produced at the weld interface and promotes themechanical mixing of the elements, which is responsible for the in-crease in volume fraction of reaction products; hence, this causesmore embrittlement of the joints with respect to the couples pro-cessed at 600 rpm tool rotational speed.

The fracture surfaces of friction stir welded joints at differenttool rotating speeds are shown in Fig. 10. From samples produceat lower tool rotating speed, it is clear that the fracture surface

S. Kundu et al. / Materials and Design 50 (2013) 370–375 375

exhibits dimples and cleavage pattern. However, the ductile areawas limited in the fracture surface of the samples processed at900 rpm. Samples processed at 1200 rpm, clearly indicates thebrittle nature of the joint by the presence of cleavage planes withdifferent alignments. The river pattern differs from grain to grain.

4. Conclusions

In the present study interstitial free steel and commerciallypure aluminium were joined by friction stir welding at 600, 900,1200 rpm tool rotational speeds and 100 mm/min traverse speed.Important conclusions are summarized below:

1. The grain sizes at the stir region of the friction stir welded jointsare finer at higher rotational speed.

2. The Al3Fe intermetallic compounds were formed at the inter-face of the friction stir welded joints. The thicknesses of inter-metallic compounds increase with the increase in the toolrotating speed due to the higher heat generated by the highrotating tool shoulder. The irregular shaped light and lightshaded particle were observed in stir zone and the light parti-cles are IF steel and the light shaded particles are Al3Fe interme-tallics compounds.

3. The micro-hardness test across the joint interface indicates thatthe micro-hardness in the joint interface is greater than thebase materials.

4. Maximum tensile strength of �123 MPa (about 86% of the CP Albase metal) along with 4.5% elongation at fracture has beenobtained for the couple processed at 600 rpm tool rotationalspeed. With an increase in the tool rotational speed, the weldstrength decreases gradually due to the increase in the thick-ness of intermetallic compounds.

Acknowledgement

The authors acknowledge the support provided by the INDO USScience & Technology Forum, New Delhi, India.

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